Hazard detection of out-of-order execution of load and store instructions in processors without using real addresses

ABSTRACT

Technical solutions are described for hazard detection of out-of-order execution of load and store instructions without using real addresses in a processing unit. An example includes an out-of-order load-store unit (LSU) for transferring data between memory and registers. The LSU detects a store-hit-load (SHL) in an out-of-order execution of instructions based only on effective addresses by: determining an effective address associated with a store instruction; determining whether a load instruction entry using said effective address is present in a load reorder queue; and indicating that a SHL has been detected based at least in part on determining that load instruction entry using said effective address is present in the load reorder queue. The LSU, in response to detecting the SHL, flushes instructions starting from a load instruction corresponding to the load instruction entry.

CROSS-REFERENCE TO PRIORITY APPLICATION

The present application is a continuation application of and claimspriority to U.S. application Ser. No. 15/726,538, filed on Oct. 6, 2017,the entire contents of which are included herein.

BACKGROUND

Embodiments of the present invention relate in general to anout-of-order (OoO) processor and more specifically to hazard detectionof out-of-order execution of load and store instructions without usingreal addresses.

In an OoO processor, an instruction sequencing unit (ISU) dispatchesinstructions to various issue queues, renames registers in support ofOoO execution, issues instructions from the various issue queues to theexecution pipelines, completes executed instructions, and handlesexception conditions. Register renaming is typically performed by mapperlogic in the ISU before the instructions are placed in their respectiveissue queues. The ISU includes one or more issue queues that containdependency matrices for tracking dependencies between instructions. Adependency matrix typically includes one row and one column for eachinstruction in the issue queue.

In the domain of central processing unit (CPU) design, and particularlyfor OoO processors, hazards pose technical challenges with theinstruction pipeline in the CPU microarchitectures when a nextinstruction cannot execute in the following clock cycle, because ofpotentially leading to incorrect computation results. Typical types ofhazards include data hazards, structural hazards, and control flowhazards (branching hazards). Data hazards occur when instructions thatexhibit data dependence modify data in different stages of a pipeline,for example read after write (RAW), write after read (WAR), and writeafter write (WAW). A structural hazard occurs when a part of theprocessor's hardware is needed by two or more instructions at the sametime, for example a memory unit being accessed both in the fetch stagewhere an instruction is retrieved from memory, and the memory stagewhere data is written and/or read from memory. Further, branchinghazards (also termed control hazards) occur with branches in thecomputer program being executed by the processor.

SUMMARY

Embodiments of the present invention include methods, systems, andcomputer program products for implementing load instruction effectiveaddress based load store unit in out of order processors.

A non-limiting example of a processing unit for executing one or moreinstructions includes a load-store unit (LSU) for transferring databetween memory and registers. The LSU detects a store-hit-load (SHL) inan out-of-order execution of instructions based only on effectiveaddresses by: determining an effective address associated with a storeinstruction; determining whether a load instruction entry using saideffective address is present in a load reorder queue; and indicatingthat a SHL has been detected based at least in part on determining thatload instruction entry using said effective address is present in theload reorder queue. The LSU, in response to detecting the SHL, flushesinstructions starting from a load instruction corresponding to the loadinstruction entry.

A non-limiting example of the computer-implemented method for executingone or more out-of-order instructions by a processing unit includesdetecting, by a load-store unit, a store-hit-load (SHL) in anout-of-order execution of instructions based only on effective addressesby: determining an effective address associated with a storeinstruction; and determining whether a load instruction entry using saideffective address is present in a load reorder queue; and indicatingthat a SHL has been detected based at least in part on determining thatload instruction entry using said effective address is present in theload reorder queue. The method also includes in response to detectingthe SHL, sending, by the load-store unit, a flush message forinstructions starting from a load instruction corresponding to the loadinstruction entry.

According to another non-limiting example, a computer program productincluding a computer readable storage medium having program instructionsembodied therewith, the program instructions executable by a processorcause the processor to perform operations including detecting, by aload-store unit, a store-hit-load (SHL) in an out-of-order execution ofinstructions based only on effective addresses by: determining aneffective address associated with a store instruction; and determiningwhether a load instruction entry using said effective address is presentin a load reorder queue; and indicating that a SHL has been detectedbased at least in part on determining that load instruction entry usingsaid effective address is present in the load reorder queue. Theoperations further include, in response to detecting the SHL, sending,by the load-store unit, a flush message for instructions starting from aload instruction corresponding to the load instruction entry.

Additional features and advantages are realized through the techniquesof the present invention. Other embodiments and aspects of the inventionare described in detail herein and are considered a part of the claimedinvention. For a better understanding of the invention with theadvantages and the features, refer to the description and to thedrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The specifics of the exclusive rights described herein are particularlypointed out and distinctly claimed in the claims at the conclusion ofthe specification. The foregoing and other features and advantages ofthe embodiments of the invention are apparent from the followingdetailed description taken in conjunction with the accompanying drawingsin which:

FIG. 1 depicts a block diagram of a system that includes an effectiveaddress based load store unit in an out of order (OoO) processor inaccordance with one or more embodiments of the present invention;

FIG. 2 is an exemplary block diagram of a processor architecture of OoOprocessor in which an effective address directory (EAD) and theassociated mechanisms for utilizing this EAD are implemented accordingto one or more embodiments of the present invention;

FIG. 3 depicts a load-store unit (LSU) of a processing core according toone or more embodiments of the present invention;

FIG. 4 is an exemplary block of an effective address directory (EAD)structure (L1 cache) in accordance with one illustrative embodiment;

FIG. 5 is an exemplary block of an effective address directory structure(L1 cache) in accordance with one illustrative embodiment;

FIG. 6 illustrates a flowchart of an example method for accessing memoryfor executing instructions by an LSU according to one or moreembodiments of the present invention;

FIG. 7 illustrates a flowchart of an example method for hazard detectionof out-of-order execution of load and store instructions in processorswithout using real addresses according to one or more embodiments of thepresent invention;

FIG. 8 illustrates a flowchart of an example method for hazard detectionof out-of-order execution of load and store instructions in processorswithout using real addresses according to one or more embodiments of thepresent invention; and

FIG. 9 depicts a block diagram of a computer system for implementingsome or all aspects of one or more embodiments of the present invention.

The diagrams depicted herein are illustrative. There can be manyvariations to the diagram or the operations described therein withoutdeparting from the spirit of the invention. For instance, the actionscan be performed in a differing order or actions can be added, deletedor modified. Also, the term “coupled” and variations thereof describeshaving a communications path between two elements and does not imply adirect connection between the elements with no interveningelements/connections between them. All of these variations areconsidered a part of the specification.

DETAILED DESCRIPTION

One or more embodiments of the present invention described hereinprovide an effective address (EA) based load store unit (LSU) for anout-of-order (OoO) processor by dynamic removal of effective realaddress table entries in the OoO processor. The technical solutionsdescribed herein use an effective address directory (EAD) in conjunctionwith an effective real table (ERT) and a synonym detection table (SDT),among other components, to facilitate reduction in chip area and furtherto improve timing of OoO processors.

As mentioned above, due to the OoO nature of modern processors, youngerload instructions may be chosen for execution ahead of older storeinstructions to a same real address (RA). When a younger loadinstruction executes ahead of an older store instruction to the same RA,the younger load instruction returns stale data and instructions in aprocessor pipeline must be flushed from the pipeline and re-fetched torestore program consistency. Typically, it is desirable to delay ayounger load instruction until an older store instruction to a same RAexecutes so that other independent instructions can execute and not beunnecessarily flushed from a processor pipeline. A step in reducingprocessor pipeline flushes due to execution of a younger loadinstruction before an older store instruction to a same RA is toidentify a load/store sequence that is to the same RA and will beexecuted OoO such that a processor pipeline flush is required.Alternatively, or in addition, the older load instruction from aload-reorder queue and all subsequent instruction therefrom are flushedfrom the load-reorder queue or from an instruction fetching unit (IFU).Flushing an instruction includes sending a flush message (that includesan appropriate identifier of the instruction) to the IFU.

Most modern computing devices provide support for virtual memory.Virtual memory is a technique by which application programs are giventhe impression that they have a contiguous working memory, or addressspace, when in fact the physical memory may be fragmented and may evenoverflow onto disk storage. Essentially, the application program isgiven a view of the memory of the computing device where the applicationaccesses a seemingly contiguous memory using an EA, in the EA spacevisible to the application, which is then translated into a physicaladdress of the actual physical memory or storage device(s) to actuallyperform the access operation. An EA is the value which is used tospecify a memory location that is to be accessed by the operation fromthe perspective of the entity, e.g., application, process, thread,interrupt handler, kernel component, etc., issuing the operation.

That is, if a computing device does not support the concept of virtualmemory, then the EA and the physical address are one and the same.However, if the computing device does support virtual memory, then theEA of the particular operation submitted by the application istranslated by the computing device's memory mapping unit into a physicaladdress which specifies the location in the physical memory or storagedevice(s) where the operation is to be performed.

Further, in modern computing devices, processors of the computingdevices use processor instruction pipelines, comprising a series of dataprocessing elements, to process instructions (operations) submitted byentities, e.g., applications, processes, etc. Instruction pipelining isa technique to increase instruction throughput by splitting theprocessing of computer instructions into a series of steps with storageat the end of each step. Instruction pipelining facilitates thecomputing device's control circuitry to issue instructions to theprocessor instruction pipeline at the processing rate of the sloweststep which is much faster than the time needed to perform all steps atonce. Processors with instruction pipelining, i.e. pipelined processors,are internally organized into stages which can semi-independently workon separate jobs. Each stage is organized and linked with a next stagein a series chain so that each stage's output is fed to another stageuntil the final stage of the pipeline.

Such pipelined processors may take the form of in-order or out-of-orderpipelined processors. For in-order pipelined processors, instructionsare executed in order such that if data is not available for theinstruction to be processed at a particular stage of the pipeline,execution of instructions through the pipeline may be stalled until thedata is available. OoO pipelined processors, on the other hand, allowthe processor to avoid stalls that occur when the data needed to performan operation are unavailable. The OoO processor instruction pipelineavoids these stalls by filling in “slots” in time with otherinstructions that are ready to be processed and then re-ordering theresults at the end of the pipeline to make it appear that theinstructions were processed in-order. The way the instructions areordered in the original computer code is known as program order, whereasin the processor they are handled in data order, i.e. the order in whichthe data and operands become available in the processor's registers.

Modern processor instruction pipelines track an instruction's EA as theinstruction flows through the instruction pipeline. It is important totrack the instruction's EA because this EA is utilized whenever theprocessing of an instruction results in the taking of an exception, theinstruction flushes to a prior state, the instruction branches to a newmemory location relative to its current memory location, or theinstruction completes its execution.

Tracking an instruction's EA is costly in terms of processor chip area,power consumption, and the like. This is because these EAs have largesizes (e.g., 64 bits) and modern processor instruction pipelines aredeep, i.e. have many stages, causing the lifetime of an instruction froman instruction fetch stage of the processor instruction pipeline to acompletion stage of the processor instruction pipeline to be very long.This cost may be further increased in highly multithreaded OoOprocessors, i.e. processors that execute instructions from multiplethreads in an OoO manner, since a vast number of instructions fromdifferent address ranges can be processing, i.e. are “in flight,” at thesame time.

In one or more examples, computing devices use a combination of pipelinelatches, a branch information queue (BIQ), and a global completion table(GCT) to track an instruction's EA. The base EA for a group ofinstructions is transferred from the front-end of the pipeline usinglatches until it can be deposited and tracked in the GCT of theinstruction sequencer unit (ISU). The number of latches needed to storethis data is on the order of the number of pipeline stages between aFetch stage and a Dispatch stage of the pipeline. This is wasteful, asthe EA is typically not needed during these stages. Rather it is simplypayload data that is “along for the ride” with the instruction group asit flows through the pipeline. In addition, this method leads toduplicate storage as branch instructions have their EAs in both the BIQand the GCT.

Accordingly, computing devices have been developed, that remove theseinefficiencies by tracking the EA solely in the GCT. For example, thesenew computing devices, an instruction sequencer unit creates an entry inthe GCT at fetch time. The EA is loaded into the GCT at this time andthen removed when the instruction completes. This eliminates manypipeline latches throughout the machine. Instead of a full EA that is aslong as number of address lines, for example a 64-bit EA, a small tag iscarried along with the instruction group through the pipeline. This tagpoints back to the entry in the GCT, which holds the base EA for thisinstruction group. Address storage in the BIQ is no longer needed asbranches can retrieve their EA directly from the GCT when they issue.Such techniques improve area efficiency, but they are not applicable inan OoO processor. Further, they lack sufficient information to processaddress requests arriving out of program order. In addition, thesetechniques cannot support dispatch and completion bandwidth required forOoO execution because they lack the ability to track instruction groupsthat may have been formed from multiple disjoint address ranges.Historically, such mechanisms have only supported instruction groupsfrom a single address range, which can significantly reduce the numberof instructions available to execute OoO. Further, to lookupcorresponding addresses, such as an RA corresponding to an EA (or viceversa) a Content Addressable Memory (CAM) is used. A CAM implementslookuptable function in a single clock cycle using dedicated comparisoncircuitry. The overall function of a CAM is to take a search word andreturn the matching memory location. However, such CAM takes chip areaas well as consumes power for such lookups.

The illustrative embodiments of technical solutions described hereinimprove upon these techniques by providing an effective addressdirectory (EAD), an effective real table (ERT), and a synonym detectiontable (SDT) that have the area efficiency of the GCT solution describedabove, but can also support a wide issue OoO pipeline while notinhibiting performance. The technical solutions described herein furtherfacilitate the processors to run with only EAs, as long as the processorcan avoid EA synonyms within an out of order (OoO) window. The OoOwindow is a set of instructions in an instruction pipeline of theprocessor. By avoiding EA synonyms in the OoO window, the processorreduces the chip area and power consumption for address translation,because the processor can avoid translation for the EA in the OoOwindow.

In other words, the technical solutions described herein address thetechnical problem by policing against EA aliasing within the OoO window,and thus reducing translation data structures and hardware for theload/store ports. Accordingly, the technical solutions described hereinfacilitate a reduction in chip area by tracking only one address, theEA. Further, the technical solutions facilitate the OoO processor to runin a 2 load and 2 store mode with partitioned load store queues, furtherreducing CAM ports that are typically used for the address translation.

For example, a processor that is capable of issuing and executinginstructions OoO may permit load instructions to be executed ahead ofstore instructions. Assuming that a real address (RA) of a younger loadinstruction does not overlap with an RA of an older store instruction,OoO execution of the younger load instruction and the older storeinstruction may provide performance advantages. In a typical program,the likelihood that an RA of a younger load instruction overlaps with anRA of an older store instruction (that executes after the younger loadinstruction) is relatively low. As is known, a store violation condition(store-hit-load (SHL)) is indicated when an RA of a store instructionhits in a load reorder queue (LRQ) of a load store unit (LSU). That is,when an RA of a newly issued older store instruction matches an RA of ayounger load instruction present in the LRQ of the LSU, an SHL isindicated. However, as detection of an SHL typically occurs late in aninstruction execution pipeline, recovering from an SHL typically resultsin a relatively severe penalty on processor performance. For example,recovery from an SHL typically involves invalidating the younger loadinstruction that caused the SHL and reissuing the younger loadinstruction, as well as all instructions issued after the older storeinstruction.

However, in case of the OoO processor, that operates using EAs (and notRAs), technical challenges are posed for detecting the SHL and LHLconditions only based on EAs, and further recovering from thesesituations. The technical solutions described herein address suchtechnical challenges by using the EA and using effective realtranslation table (ERT) indices stored with EAD entries. The technicalsolutions thus facilitate the OoO processor have lesser chip areadedicated to storing and manipulating real addresses.

Turning now to FIG. 1, a block diagram of a system 100 that includes aninstruction sequencing unit (ISU) of an OoO processor for implementingthe technical solutions for avoiding EA synonyms in an OoO instructionwindow is generally shown according to one or more embodiments of thepresent invention. The system 100 shown in FIG. 1 includes aninstruction fetch unit/instruction decode unit (IFU/IDU) 106 thatfetches and decodes instructions for input to a setup block 108 whichprepares the decoded instructions for input to a mapper 110 of the ISU.In accordance with one or more embodiments of the present invention, sixinstructions at a time from a thread can be fetched and decoded by theIFU/IDU 106. In accordance with one or more embodiments of the presentinvention, the six instructions sent to the setup block 108 can includesix non-branch instructions, five non-branch instructions and one branchinstruction, or four non-branch instructions and two branchinstructions. In accordance with one or more embodiments of the presentinvention, the setup block 108 checks that sufficient resources such asentries in the issue queues, completion table, mappers and registerfiles exist before transmitting the fetched instructions to these blocksin the ISU.

The mappers 110 shown in FIG. 1 map programmer instructions (e.g.,logical register names) to physical resources of the processor (e.g.,physical register addresses). A variety of mappers 110 are shown in FIG.1 including a condition register (CR) mapper; a link/count (LNK/CNT)register mapper; an integer exception register (XER) mapper; a unifiedmapper (UMapper) for mapping general purpose registers (GPRs) andvector-scalar registers (VSRs); an architected mapper (ARCH Mapper) formapping GPRs and VSRs; and, a floating point status and control register(FPSCR) mapper.

The output from the setup block 108 is also input to a global completiontable (GCT) 112 for tracking all of the instructions currently in theISU. The output from the setup block 108 is also input to a dispatchunit 114 for dispatching the instructions to an issue queue. Theembodiment of the ISU shown in FIG. 1 includes a CR issue queue, CRISQ116, which receives and tracks instructions from the CR mapper andissues 120 them to instruction fetch unit (IFU) 124 to execute CRlogical instructions and movement instructions. Also shown in FIG. 1 isa branch issue queue, Branch ISQ 118, which receives and tracks branchinstructions and LNK/CNT physical addresses from the LNK/CNT mapper.Branch ISQ 118 can issue an instruction to IFU 124 to redirectinstruction fetching if a predicted branch address and/or direction wasincorrect.

Instructions output from the dispatch logic and renamed registers fromthe LNK/CNT mapper, XER mapper, UMapper (GPR/VSR), ARCH Mapper(GPR/VSR), and FPSCR mapper are input to issue queue 102. As shown inFIG. 1, issue queue 102 tracks dispatched fixed point instructions (Fx),load instructions (L), store instructions (S), and vector-and-scalerunit (VSU) instructions. As shown in the embodiment of FIG. 1, issuequeue 102 is broken up into two parts, ISQ0 1020 and ISQ1 1021, eachportion holding N/2 instructions. When the processor is executing insingle threaded (ST) mode, the issue queue 102 can be used as a singlelogical issue queue that contains both ISQ0 1020 and ISQ1 1021 toprocess all of the instructions (in this example all N instructions) ofa single thread.

When the processor is executing in multi-threaded (MT) mode, ISQ0 1020can be used to process N/2 instructions from a first thread and ISQ11021 is used to process N/2 instructions from a second thread ISQ1 1021.

As shown in FIG. 1, issue queue 102 issues instructions to executionunits 104 which are split into two groups of execution units,1040 and1041. Both groups of execution units, 1040 and 1041, that are shown inFIG. 1, include a full fixed point execution unit (Full FX0, Full FX1);a load execution unit (LU0, LU1); a simple fixed point, store data, andstore address execution unit (Simple FX0/STDO/STA0, SimpleFX1/STD1/STA1); and a floating point, vector multimedia extension,decimal floating point, and store data execution unit (FP/VMX/DFP/STD0,FP/VMX/DFP/STD1). Collectively, the LU0, the Simple FX0/STD0/STA0, andthe FP/VMX/DFP/STD0 form a load-store unit (LSU) 1042. Similarly, theLU1, the Simple FX1/STD1/STA1, and the FP/VMX/DFP/STD1 form a load-storeunit (LSU) 1043. The two LSUs 1042 and 1043 together are referred to asan LSU of the system 100.

As shown in FIG. 1, when the processor is executing in ST mode, thefirst group of execution units 1040 execute instructions issued fromISQ0 1020 and the second group of execution units 1041 executeinstructions issued from ISQ1 1021. In alternate embodiments of thepresent invention when the processor is executing in ST mode,instructions issued from both ISQ0 1020 and ISQ1 1021 in issue queue 102can be issued to execution units in any of the execution units 1040 inthe first group of execution units 1040 and the second group ofexecution units 1041.

In accordance with one or more embodiments of the present invention,when the processor is executing in MT mode, the first group of executionunits 1040 execute instructions of the first thread issued from ISQ01020 and the second group of execution units 1041 execute instructionsof the second thread issued from ISQ1 1021.

The number of entries in the issue queue 102 and sizes of other elements(e.g., bus widths, queue sizes) shown in FIG. 1 are intended to beexemplary in nature as embodiments of the present invention can beimplemented for issue queues and other elements of a variety ofdifferent sizes. In accordance with one or more embodiments of thepresent invention, the sizes are selectable, or programmable.

In one or more examples, the system 100, in accordance with theillustrative embodiments, is an OoO processor. FIG. 2 is an exemplaryblock diagram of a processor architecture of OoO processor in which anEAD and the associated mechanisms for utilizing this EAD are implementedaccording to one or more embodiments of the present invention. As shownin FIG. 2, the processor architecture includes an instruction cache 202,an instruction fetch buffer 204, an instruction decode unit 206, and aninstruction dispatch unit 208. Instructions are fetched by theinstruction fetch buffer 204 from the instruction cache 202 and providedto the instruction decode unit 206. The instruction decode unit 206decodes the instruction and provides the decoded instruction to theinstruction dispatch unit 208. The output of the instruction dispatchunit 208 is provided to the global completion table 210 and one or moreof the branch issue queue 212, the condition register issue queue 214,the unified issue queue 216, the load reorder queue 218, and/or thestore reorder queue 220, depending upon the instruction type. Theinstruction type is determined through the decoding and mapping of theinstruction decode unit 206. The issue queues 212-220 provide inputs tovarious ones of execution units 222-240. The data cache 250, and theregister files contained with each respective unit, provides the datafor use with the instructions.

The instruction cache 202 receives instructions from the L2 cache 260via the second level translation unit 262 and pre-decode unit 270. Thesecond level translation unit 262 uses its associate segment look-asidebuffer 264 and translation look-aside buffer 266 to translate addressesof the fetched instruction from effective addresses to system memoryaddresses. The pre-decode unit partially decodes instructions arrivingfrom the L2 cache and augments them with unique identifying informationthat simplifies the work of the downstream instruction decoders.

The instructions fetched into the instruction fetch buffer 204 are alsoprovided to the branch prediction unit 280 if the instruction is abranch instruction. The branch prediction unit 280 includes a branchhistory table 282, return stack 284, and count cache 286. These elementspredict the next EA that should be fetched from the instruction cache. Abranch instruction is a point in a computer program where flow ofcontrol is altered. It is the low-level machine instruction that isgenerated from control constructs in a computer program, such asif-then-else or do-while statements. A branch can be not taken, in whichthe flow of control is unchanged and the next instruction to be executedis the instruction immediately following it in memory, or it can betaken, in which the next instruction to be executed is an instruction atsome other place in memory. If the branch is taken, a new EA needs to bepresented to the instruction cache.

The EA and associated prediction information from the branch predictionunit are written into an effective address directory 290. This EA islater confirmed by the branch execution unit 222. If correct, the EAremains in the directory until all instructions from this address regionhave completed their execution. If incorrect, the branch execution unitflushes out the address and the corrected address is written in itsplace. The EAD 290 also includes a logic unit that facilitates using thedirectory as a CAM.

Instructions that read from or write to memory (such as load or storeinstructions) are issued to the LS/EX execution unit 238, 240. The LS/EXexecution unit retrieves data from the data cache 250 using a memoryaddress specified by the instruction. This address is an EA and needs tofirst be translated to a system memory address via the second leveltranslation unit before being used. If an address is not found in thedata cache, the load miss queue is used to manage the miss request tothe L2 cache. In order to reduce the penalty for such cache misses, theadvanced data prefetch engine predicts the addresses that are likely tobe used by instructions in the near future. In this manner, data willlikely already be in the data cache when an instruction needs it,thereby preventing a long latency miss request to the L2 cache.

The LS/EX execution unit 238, 240 executes instructions out of programorder by tracking instruction ages and memory dependences in the loadreorder queue 218 and store reorder queue 220. These queues are used todetect when OoO execution generated a result that is not consistent withan in-order execution of the same program. In such cases, the currentprogram flow is flushed and performed again.

The processor architecture further includes the EAD 290 which maintainsthe effective address of a group of instructions in a centralized mannersuch that the EA is available when needed but is not required to bepassed through the pipeline. Moreover, the EAD 290 includes circuitryand/or logic for supporting OoO processing. FIG. 2 shows the EAD 290being accessed via the branch prediction unit 280, however, it should beappreciated that circuitry may be provided for allowing various ones ofthe units shown in FIG. 2 to access the EAD 290 without having to gothrough the branch prediction unit 280.

Those of ordinary skill in the art will appreciate that the hardware inFIGS. 1-2 may vary depending on the implementation. Other internalhardware or peripheral devices, such as flash memory, equivalentnon-volatile memory, or optical disk drives and the like, may be used inaddition to or in place of the hardware depicted in FIGS. 1-2. Inaddition, the processes of the illustrative embodiments may be appliedto a multiprocessor data processing system, other than the SMP systemmentioned previously, without departing from the spirit and scope of thepresent invention.

Moreover, the data processing system 100 may take the form of any of anumber of different data processing systems including client computingdevices, server computing devices, a tablet computer, laptop computer,telephone or other communication device, a personal digital assistant(PDA), or the like. In some illustrative examples, data processingsystem 100 may be a portable computing device configured with flashmemory to provide non-volatile memory for storing operating system filesand/or user-generated data, for example. Essentially, data processingsystem 100 may be any known or later developed data processing systemwithout architectural limitation.

As will be appreciated by one skilled in the art, the present inventionmay be embodied as a system, apparatus, or method. In one illustrativeembodiment, the mechanisms are provided entirely in hardware, e.g.,circuitry, hardware modules or units, etc. of a processor. However, inother illustrative embodiments, a combination of software and hardwaremay be utilized to provide or implement the features and mechanisms ofthe illustrative embodiments. The software may be provided, for example,in firmware, resident software, micro-code, or the like. The variousflowcharts set forth hereafter provide an outline of operations that maybe performed by this hardware and/or combination of hardware andsoftware.

In illustrative embodiments in which the mechanisms of the illustrativeembodiments are at least partially implemented in software, anycombination of one or more computer usable or computer readablemedium(s) that store this software may be utilized. The computer-usableor computer-readable medium may be, for example, but not limited to, anelectronic, magnetic, optical, electromagnetic, infrared, orsemiconductor system, apparatus, or device. More specific examples (anon-exhaustive list) of the computer-readable medium would include thefollowing: a random access memory (RAM), a read-only memory (ROM), anerasable programmable read-only memory (EPROM or Flash memory), etc.

Typically, for every load and every store instruction, an EA isconverted to corresponding RA. Such an EA to RA conversion is alsoperformed for an instruction fetch (I-fetch). Such conversion typicallyrequired an effective to real address table (ERAT) for retrieval ofinstructions from lower order memory. In the technical solutionsdescribed herein, the EA to RA conversion is not performed for everyload and store instruction, rather only in case of load-misses, I-Fetchmisses, and all stores.

By using only EA for the operations, the technical solutions facilitateremoval of RA bits (for example, bits 8:51) from one or more datastructures, such as an EA directory (also referred to as L1 directory),LRQF entries, LMQ entries. Further, SRQ LHS RA compare logic is notexecuted if only the EA is being used. Removing such elements reduceschip area of the processor used, thus facilitating a reduction in chiparea over typical processors.

Further, by only using EA, the technical solutions herein eliminate ERATcamming on every load and store address generation. The technicalsolutions further eliminate RA bus switching throughout the unit, andalso avoids fast SRQ LHS RA cam. The technical solutions thus facilitatethe processor to consume lesser power compared to typical processors bynot performing the above operations.

Further yet, the technical solutions herein also facilitate improvementsto L1 latency. For example, the technical solutions herein, byeliminating the address conversions are at least 1 cycle faster indetermining “final dval” compared to typical processors that perform theEA to RA conversions. The latency is also improved because by using onlythe EA (without RA conversion) eliminates “bad dval” condition(s), suchas set predict array multi-hit, setup table hit/RA miss, and the like.In a similar manner, the technical solutions herein facilitateimprovement to L2 latency.

The technical challenges of using only an EA-based LSU include that outof order execution of the instructions may lead to hazards (LHL, SHL,LHS), and such hazards are to be detected without using correspondingreal addresses for an EA only implementation. The technical solutionsdescribed herein address such technical problem. The RA is not used forload-hit-store, store-hit-load or load-hit-load type of OoO executionhazard detection as it used typically. The RA calculation for storeshappens before the store is completed, because after completion anyinterrupt for the store instructions are not processed (store cangenerate an address translation related interrupt, which needs to beprocessed before the store is completed). Here the RA calculation isdone when the store is issued (and not when drained). The technicalsolutions described herein determine LHS (load-hit-store), SHL(store-hit-load) and LHL (load-hit-load) based on the EA and theERT-Index stored with the EA directory entries, and also using an loadreorder queue (LRQF) with structure and operation as described herein.

Referring again to the figures, FIG. 3 depicts a load-store unit (LSU)104 of a processing core according to one or more embodiments of thepresent invention. The LSU 104 depicted facilitates execution in a 2load 2 store mode; however, it should be noted that the technicalsolutions described herein are not limited to such an LSU. The executionflow for the LSU is described below. From the load or store instructionsthe EA (as used by the programmer in a computer program) is generated.Similarly, for instruction fetch also an EA is generated. Typically, theEA was converted to RA (Real Address, as used by the hardware, afterEA-to-RA translation) for every instruction, which required larger chiparea, and frequent translations, among other technical challenges. Thetechnical solutions described herein address such technical challengesby using only the EA (without translation to RA), and using an effectivereal table (ERT) 255 to generate the RA, only on load-misses, I-Fetchmisses and stores.

The LSU 104 includes a load-reorder-queue (LRQF) 218, where all loadoperations are tracked from dispatch to complete. The LSU 104 furtherincludes a second load-reorder-queue LRQE 225. When a load is rejected(for cache miss, or translation miss, or previous instruction it dependson got rejected) the load is taken out of the issue queue and placed ina LRQE entry for it to be re-issued from there. The depicted LRQE 225 ispartitioned into 2 instances, LRQEO, and LRQE1 for the two load mode,with 12 entries each (24 entries total). In ST mode, no threads/pipebased partition exists. In the MT mode, T0, T2 operations launched onpipe LD0; and T1, T3 operations launched on pipe LD1, for relaunch.

As depicted, the LRQF 218 is partitioned into 2 instances LRQF0 andLRQF1 for the two load mode, with 40 entries (each instance). The LRQF218 is circular in order entry allocation, circular in order entrydrain, and circular in order entry deallocation. Further, in MT mode,T0, T2 operations launched on pipes LD0, ST0; and T1, T3 operationslaunched on pipes LD1, ST1. In ST mode, the LRQF does not have anypipes/threads.

In one or more examples, the LRQF 218 (and other structures describedherein) is partitioned for the SMT4 mode as TO : LRQF0[0:19] circularqueue, T1: LRQF1[0:19] circular queue; and T2 : LRQF0[20:39] circularqueue, T3: LRQF1[20:39] circular queue.

In one or more examples, the LRQF 218 (and other structures describedherein) is partitioned for the SMT2 mode as T0: LRQF0[0:39] circularqueue, and T1: LRQF1[0:39] circular queue. Further, in one or moreexamples, for the ST mode, LRQF0[0:39] circular queue, with the LRQF1being an LRQF0 copy. For other data structure, a similar partitionpattern is used with the second instance being a copy of the firstinstance in ST mode.

In case of a cross invalidation flush (XI flush), for the LRQF, NTC+1flush any thread that an XI or store drain from another thread hits sothat explicit L/L ordering flushes on sync's is not performed by the LSU104 in case of the XI flush.

All stores check against the LRQF 218 for SHL detection, upon which theLRQF 218 initiates a flush of any load, or everything (anyinstruction/operation) after the store. Further, DCB instructions checkagainst the LRQF 218 for SHL cases, upon which the LRQF 218 causes aflush of the load, or everything after the DCB. Further, all loads checkagainst the LRQF 218 for LHL detection (sequential load consistency),upon which the :RQF 218 causes a flush of younger load, or everythingafter the older load. In one or more examples, the LRQF 218 providesquad-word atomicity, and the LQ checks against the LRQF 218 for quadatomicity and flushes LQ if not atomic. Further yet, in case of LARXinstructions, the LSU 104 checks against the LRQF 218 for larx-hit-larxcases, and in response flushes younger LARX, or everything after theolder larx instruction.

Thus, the LRQF 218 facilitates tracking all load operations from issueto completion. Entries in the LRQF 218 are indexed with Real_Ltag(rltag), which is the physical location in the queue structure. The ageof a load operation/entry in the LRQF 218 is determined with aVirtual_Ltag (vltag), which is in-order. The LRQF flushes a load usingGMASK and partial group flush using GTAG and IMASK. The LRQF logic canflush from current iTag or iTag+1 or precise load iTag.

Further yet, the LRQF does not include an RA (8:51) field typicallyused, and instead is EA-based and includes an ERT ID (0:6), andEA(40:51) (saving of 24 bits). The LRQF page match on SHL, LHL is basedon ERT ID match. Further, Each LRQ entry has a “Force Page Match” bit.When an ERT ID is invalidated that matches the LRQ Entry ERT ID theForce Page Match bit is set. The LRQ will detect LHL, SHL, and storeordering flushes involving any entry with Force Match Match=1.

The SRQ 220 of the LSU 104 has similar structure as the LRQF 218, withtwo instances SRQR0 and SRQR1 of 40 entries (each instance), which arecircular in order entry allocation, circular in order entry drain, andcircular in order entry deallocation. Further, the SRQ 220 ispartitioned similar to the LRQ 218, for example T0, T2 ops launched onpipes LD0, ST0; T1, T3 ops launched on pipes LD1, ST1; and nopipe/thread partition in ST mode. In the ST mode, both copies haveidentical values, with the copies being different in the MT modes. InSMT4 mode, both instances are further partitioned, with each threadallocated 20 entries from the SRQ 220 (see example partition for LRQFdescribed herein). In one or more examples, for store drain arbitration,an intra-SRQ read pointer multiplexing is performed in the SMT4 mode.Alternatively, or in addition, an inter SRQ0/1 multiplexing is performedin SMT2, and SMT4 modes. In the ST mode drain is performed only on SRQ0.

Each entry of the SRQ 220 contains a store TID(0:1), an ERT_ID(0:6),EA(44:63), and RA(8:51). To detect LHS, the LSU uses the {Store Tid,EA(44:63)}, thus eliminating RA LHS alias check. The ERT_ID is used to“catch” EA(44:63) partial match mis-speculation. The SRQ entry has theRA(8:51), which is translated at store agen, and is only used whensending store requests to the L2 (store instruction drained, notissued). Each SRQ entry also has a “Force Page Match” bit. The forcepage match bit is set when an ERT ID is invalidated that matches the SRQentry ERT ID. The SRQ can detect LHS involving any entry with Force PageMatch=1. For example, LHS against an entry with Force Page Match=1causes a reject of the load instruction. Further, a store drain forces amiss in the L1 cache if Force Page Match=1 for the SRQ entry. This worksin tandem with “Extended store hit reload” LMQ actions.

For example, for an LMQ, an LMQ Address Match={ERT ID, EA PageOffset(xx:51), EA(2:56)} match. Further, a “Force Page Match” bit ofeach LMQ entry is set (=1) when an ERT_ID is invalidated that matchesthe LMQ Entry ERT ID. The LMQ rejects a load miss if a valid LMQentry[x]ForcePageMatch=1 and Ld Miss EA[52:56]=LMQEntry[X]EA(52:56).Further, the LMQ has an extended store hit reload. For example, LMQsuppresses reload enable if Reload EA(52:56)=SRQEntry[X] EA(52:56) andSRQEntry[X]ForcePageMatch=1. Alternatively, or in addition, LMQsuppresses reload enable if LMQEntry[X]EA(52:56)=StDrain EA(52:56) andStDrainForcePageMatch=1.

The LSU 104 depicted collapses a Store Data Queue (SDQ) as part of theSRQ 220 itself to further save chip area. The operands are stored in anentry of the SRQ itself if the operand size is less than the SRQ entrysize, for example 8 bytes. In case of wider operands, such as vectoroperands, for example are 16 bytes wide, the SRQ stores the operandsusing two consecutive entries in the SRQ 220 in MT mode. In ST mode, thewider operands are stored in the SRQ0 and SRQ1, for example 8 byteseach.

The SRQ 220 queues operations of type stores, barriers, DCB, ICBI or TLBtype of operations. A single s-tag is used for both store_agen andstore_data. The SRQ 220 handles load-hit-store (LHS) cases (same threadonly). For example, all loads issued are checked by the SRQ 220 toensure there is no older stores with a data conflict. For example, thedata conflict is detected by comparing loads EA and data byte flagsagainst older stores in the SRQ EA array.

SRQ entries are allocated at dispatch where the dispatched instructiontags (itags) are filled into the correct row. Further, SRQ entries aredeallocated on store drain. In one or more examples, the itag arrayshold “overflow” dispatches. For example, information is written into theitag array at dispatch if the row in the SRQ that is desired, say SRQentry x is still in use. When, the SRQ entry x is deallocated, itscorresponding row in the SRQ overflow itag structure is read out andcopied into the main SRQ itag array structure (read of the overflow itagstructure gated by whether there are any valid entries in the overflowitag array for a given thread/region). The main SRQ 0/1 itag array iscammed (or ½ cammed in SMT4) to determine which physical row to writeinto upon store issue, so that the ISU issues stores based on the itag.The SRQ 220 sends to the ISU, the itag when a store drains &deallocates.

ISU assigns virtual sub-regions to store dispatches to avoid overlappingissues. For example, in ST mode, the ISU does not issue a virtual SRQentry 40 until real SRQ entry 0 is deallocated by an entry 0 storedrain. Further, in SMT4 mode, the ISU cannot issue Tx virtual SRQ entry20 until real Tx SRQ entry 0 is drained and deallocated. The ISUsubdivide each thread partition further into 4 regions.

For example, for ST mode subdivide the SRQ 220 into 4 subregions: PingA: SRQ entries 0-9, Ping B: SRQ entries 10-19, Ping C: SRQ entries20-29, Ping D: SRQ entries 30-39; and Pong A: SRQ entries 0-9, Pong B:SRQ entries 10-19, Pong C: SRQ entries 20-29, Pong D: SRQ entries 30-39.Initially, ISU issues Ping A,B,C,D itags. Further, the ISU does notissue Pong A itags until Ping A itags have been deallocated.Subsequently, after all Ping A itags are deallocated, ISU issues Pong Aitags, but not issue Pong B itags until Ping B itags have beendeallocated, similar to the case A. In one or more examples, the ISUholds 3 extra bits in the issue queue entry (1 wrap bit +2 bits todelineate which subregion) to create a pseudo issue dependency based onsubregion.

FIG. 4 is an exemplary block of an effective address directory structure(L1 cache) 290 in accordance with one illustrative embodiment. In one ormore examples, the EAD is part of the LSU 104. As shown in FIG. 3, theEAD 290 is comprised of one or more entries, e.g., entry 0 to entry N,with each entry comprising a plurality of fields of informationregarding a group of one or more instructions. For example, in oneillustrative embodiment, each entry in the EAD 290 may represent between1 and 32 instructions. Entries in the EAD 290 are created in response toa fetch of an instruction that is in a new cache line of the processorcache, e.g., the L2 cache 260 in FIG. 2. The entry in the EAD 290 isupdated as additional instructions are fetched from the cache line. Eachentry of the EAD 290 is terminated on a taken branch (i.e. a fetchedbranch instruction from the cache is resolved as “taken”), cache linecrossing (i.e. the next fetched instruction is in a different cache linefrom the current cache line), or a flush of the processor pipeline (suchas when a branch misprediction occurs or the like).

As shown in FIG. 3, the fields of the EAD 290 entry comprise a base EA310, a first instruction identifier 320, a last instruction identifier330, a closed identifier 340, a global history vector field 350, a linkstack pointer field 360, a branch taken identifier 370, and a branchinformation field 380. A head pointer 390 points to the head, oryoungest entry, in the EAD 290. A tail pointer 392 points to the tail,or oldest entry, in the EAD 290. In addition, a tail+1 pointer 394 isprovided for pointing to the entry in the EAD 290 that is second to thetail, or oldest, entry in the EAD 290.

The base EA 310 is the starting EA of the group of instructions. Eachinstruction in the group of instructions has the same base EA and thenan offset from it. For example, in one illustrative embodiment, the EAis a 64 bit address comprising bits 0:63. The base EA may comprise, inone illustrative embodiment, bits 0:56 of this EA with bits 57:61representing the offset from the base EA for the specific instructionwithin the group of instructions. Bits 62 and 63 point to a specificbyte of each instruction. In the illustrative embodiment, each addressreferences an instruction that is 32 bits long (i.e. 4 bytes), whereeach byte in memory is addressable. An instruction cannot be furtherdivided into addressable subcomponents, and thus an instruction addresswill always have bits 62 and 63 set to zero. Therefore, bits 62 and 63do not need to be stored and can always be assumed to be zero by theEAD.

The first instruction identifier field 320 stores the EA offset bits,e.g., bits 57:61 of the EA for the first instruction in the group ofinstructions to which the EAD 290 entry corresponds. A combination ofthe base EA from field 310 and the EA offset bits in the firstinstruction identifier field 320 provides the EA for the firstinstruction in the group of instructions represented by the EAD 290entry. This first field 320 may be used, as discussed hereafter, forrecovering a refetch address and branch prediction information in theevent that the pipeline is flushed, for example.

The last instruction identifier field 330 stores the EA offset bits,e.g., bits 57:61 of the EA, for the last instruction in the group ofinstructions to which the EAD 290 entry corresponds. EAD logic updatesthis field as additional instructions in the group of instructionsrepresented by the EAD 290 entry are fetched. The EAD logic discontinuesupdating of this field 330 in the particular EAD 290 entry in responseto the EAD 290 entry being closed when a cache line crossing or takenbranch is found. This field will remain intact unless a pipeline flushoccurs that clears out a portion of the EAD entry. In such cases, theEAD logic updates this field to store the EA offset bits of theinstruction that is now the new last instruction in the entry as aresult of the flush. This field is ultimately used for completion, asdiscussed hereafter, to release the entry in the EAD 290.

The closed identifier field 340 is used to indicate that the EAD 290entry has been closed and no more instruction fetches will be made tofetch instructions for the instruction group corresponding to the EAD290 entry. An EAD 290 entry may be closed for a variety of differentreasons, including a cache line crossing, a branch being taken, or aflush of the pipeline. Any of these conditions may result in the valuein the closed field 340 being set to indicate the EAD entry is closed,e.g., set to a value of “1.” This field 340 is used at completion torelease an entry in the EAD 290, as discussed in greater detailhereafter.

The global history vector field 350 identifies the global history vectorfor the first instruction fetch group that created the entry in the EAD290. The global history vector is used to identify a history of whetherbranches were taken or not taken, as discussed in greater detailhereafter. The global history vector is used for branch predictionpurposes to help in determining, based on the recent history of branchesbeing taken or not taken, whether a current branch is likely to be takenor not.

The link stack pointer field 360 identifies the link stack pointer forthe first instruction fetch group that created the entry in the EAD 290.The link stack pointer is another branch prediction mechanism that willbe described in greater detail hereafter.

The branch taken field 370 indicates whether the group of instructionscorresponding to the EAD 290 entry had a branch instruction in which thebranch was taken. The value in the branch taken field 370 is updated inresponse to a branch instruction of the instruction group represented bythe EAD 290 entry being predicted as taken. In addition, once a branchin the instructions of the EAD 290 entry is taken, the EAD 290 entry isalso closed by writing the appropriate value to the closed field 340.Since the branch taken field is written speculatively at predictiontime, it may need to be replaced with the correct value when the branchis actually executed. For example, a branch could be predicted as nottaken, in which case a “0” would be written into the branch taken field.However, later in execution, the branch could be found to be taken, inwhich case the field must be corrected by writing it to a value of “1”.The second write only occurs if the branch was mispredicted.

The branch information field 380 stores miscellaneous branch informationthat is used for updating branch prediction structures when a branchresolves, or architected EA state when a branch instruction completes.

The ERT_ID field 385 stores an index into the ERT table (describedfurther), which identifies a corresponding ERT entry. When an ERT entryis invalidated, the associated ERT_ID is invalidated and it will alsoinvalidate all associated entries in L1 cache and L1 D cache.

Entries in the EAD 290 are accessed using an effective address tag(eatag) that comprises at least two parts: base eatag and an eatagoffset. In one illustrative embodiment, this eatag is a 10 bit value,which is relatively much smaller than the 64 bit EA. With a 10 bit eatagvalue, and a EAD 290 having a size of 14 entries, in one exemplaryimplementation, the eatag is comprised of a first 5 bits, referred to asthe base eatag, for identifying an entry within the EAD 290 and a second5 bits, referred to as the eatag offset, for providing the offset of thespecific instruction within the group of instructions represented by theentry in the EAD 290. A first bit in the 5 bits identifying the entrywithin the EAD 290 may be used as a wrap bit to indicate whether a wrapoccurred when going from the topmost entry to the bottom most entry ofthe EAD 290. This may be used for age detection. The second throughfifth bits of the 5 bits identifying the entry within the EAD 290 may beused to index into the EAD to identify the base EA of the instruction,i.e. EA(0:56). The 5 bit offset value may be used to provide, forexample, bits 57:61 of the particular instruction's EA. This exampleeatag is illustrated below:

-   -   eatag(0:9)=row(0:4)∥offset(0:4)    -   row(0): Wrap bit for the EAD indicating whether or not a wrap        occurred    -   when going from the topmost entry to bottom most entry of the        EAD.    -   row(1:4): Index into 14-entry EAD used to determine EA(0:56) of        the instruction.    -   offset(0:4): Bits 57:61 of the instruction's EA.

FIG. 4 depicts an example effective real table (ERT) structure accordingto one or more embodiments of the present invention. In one or moreexamples, the ERT 255 includes 128 total entries, however it should benoted that the total number of entries can be different in otherexamples, and further that the number of entries may be selectable orprogrammable. Further, in case the LSU executes two instructions viaseparate threads in parallel, the LSU maintains two instances of the ERT255 with 64 (half) entries each, for example an ERT0 and an ERT1. Thedescription below describes any one of these instances, unless specifiedotherwise.

The ERT 255 includes a valid ERT entry, in general, exists for any pageactive in the L1 I-Cache or D-Cache directory (EAD 290) or an SRQ entryor an LRQF entry or an LMQ entry. In other words, ERT 255 is a table ofall active RPN's in the LSU and IFU (L1 DC, SRQ, LRQE, LRQF, LMQ, IC).In one or more examples, if the processor 106 is operating in ST mode,all entries in the ERT 255 are used for the single thread that is beingexecuted. Alternatively, in one or more examples, the entries in the ERT255 are divided into sets, and in ST mode, each set has the samecontent. For example, if the ERT 255 has 128 total entries, and supportsmaximum two threads, when the processor operates in ST mode, the ERT 255includes two sets of 64 entries each, and the two sets have the samecontent.

Alternatively, if the processor 106 is operating in the MT mode, the ERTentries are divided among the threads being executed. For example, incase of two threads, the ERT entries are divided into two equal sets, afirst set of entries associated with a first thread, and a second set ofentries associated with a second thread. For example, 1 copy for LD0pipe L1 misses, ST0 pipe launches, T0/T2 I-Fetches: ERT0, which handlesT0 in SMT2 mode and T0/T2 in SMT4 mode; and 1 copy for LD1 pipe L1misses, ST1 pipe launches, T1/T3 I-Fetches: ERT1, which handles T1 inSMT2 mode and T1/T3 in SMT4 mode.

In one or more examples, each ERT entry includes at least the followingfields ERT fields, ERT_ID (0:6), Tid_en (0:1), Page Size (0:1), EA(0:51), and RA (8:51). The ERT_ID field is a unique index for each ERTentry. For example, the ERT_ID may include a sequential number thatidentifies the ERT entry. The ERT_ID is stored in the ERT_ID field 285of the EAD 290, and other data structures used by the LSU. The TID_enfield indicates if the entry is enabled for being used in MT mode, andin one or more examples a the thread identifier of the instruction thatis using the ERT entry. Further the Page Size indicates the memory pagesize to which the ERT entry refers. The RA includes a real addressassociated with the ERT entry.

The LSU refers to the ERT 255 only in cases where the RA is to be usedfor completing execution of an instruction. As described herein, the ERT255 is consulted by the LSU for the following four functions, 1. Ifetch,Load or store missing the L1 cache; 2. stores from another thread withinthe core; 3. Snoop (XI) from another core; and 4. TLB and SLBinvalidation.

In the first case of Ifetch, Load or store missing the L1 cache, the EAand thread_id are used to index into the ERT 255 and the RA from thecorresponding ERT entry is sent to the L2 cache if a valid ERT entryexists. In case of an ERT miss, that is a valid ERT entry does not existfor the EA and thread_id, the SLB/TLB is used.

In the second case, where stores from another thread within the core, astore drained from the SRQ checks the ERT 255 and ERTE table (describedfurther) for a hit from another thread. If there is no hit from adifferent thread, then there is no load from another thread that isusing the same RA. If there is a hit from a different thread using thesame RA, the LSU checks the LRQ. Although, rare, in case a hit fromanother thread exists if the RA is used by another thread(s).Accordingly, the LSU looks up the ERT table 400 to find the relevantEA(s) for the common RA. The EA(s) are then used to look into the LRQfor a match (reject any store issue in that cycle). LRQ is partitionedper thread, so the LSU only looks into relevant thread's LRQ. If thereis matching load(s) in the LRQ, the LSU flushes the oldest of thematching load(s).

In the third case of a snoop from another core of the processor, the LSUworks similar to the second case, and checks for a hit from any of theother threads being executed. In case the TLB/SLB are invalidated, theERT 255 is also invalidated.

The LRQF 218 in conjunction with the ERT table 400 is used for detectingand handling hazards, such as LHL, SHL, and LHS. For example, each storeinstruction is checked against the LRQF 218 for SHL detection, and incase a load instruction exists in the LRQF 218 for the same EA as thestore instruction, the store instruction and other entries therefrom inthe IFU are flushed, or the load instruction from the LRQF 218 isflushed. Further, in case of DCB instructions, the instructions arechecked against the LRQF 218 for SHL cases and a load and everythingafter the DCB are flushed, or the load is flushed. For each data setthat the system 100 wants to process, a corresponding data control block(DCB) and data definition (DD) statement or its dynamic allocationequivalent is to be configured.

Further, in one or more examples, as data are moved into and out ofphysical storage locations in system memory (e.g., in response to theinvocation of a new process or a context switch), the entries in the TLBare updated to reflect the presence of the new data, and the TLB entriesassociated with data removed from system memory (e.g., paged out tononvolatile mass storage) must be invalidated. Typically, theinvalidation of TLB entries is the responsibility of software and isaccomplished through the execution of an explicit TLB invalidate entryinstruction (e.g., TLBIE in the POWER™ instruction set architecture(ISA)). The LRQF 218 provides TLBIE support by facilitating a TLBIEinstruction to mark the entries in the LRQF 218, where the LRQF 218indicates if any valid entries are marked.

Further, each load instruction is checked against the LRQF 218 for LHLdetection (sequential load consistency), and in case of an LHL, theyounger load instruction is flushed, or everything after the older loadis flushed.

Further yet, each LARX instruction is checked against the LRQF 218 forlarx-hit-larx cases, and in case the situation is detected, the youngerLARX instruction is fluedh, or everything after the older LARX isflushed.

The technical solutions described herein thus facilitate hazarddetection using only EA and without EA-to-RA translation for every loadand store path, which is more expensive in terms of time as well as chiparea (to store RA and CAM ports for translation). Further, the technicalsolutions facilitate an improved timing to detect SHL and suppress DVALin time.

FIG. 6 illustrates a flowchart of an example method for accessing memoryfor executing instructions by an LSU according to one or moreembodiments of the present invention. The instruction may be a load, astore, or an instruction fetch for the OoO processor 106. Upon receivingthe instruction, the LSU uses parameters of the instruction to check ifthe EAD 290 has an entry corresponding to the instruction, as shown at505, and 510. In one or more examples, the parameters used for checkinginclude thread identifier, page size, EA, among others.

If the LSU experiences an EAD hit in the EAD 290, that is the EA of theinstruction matches an entry in the EAD table 300, the LSU reads thecontents of the matching EAD entry to determine a corresponding ERTentry, as shown at 520. Each EAD entry contains the ERT_ID (0:6) field285. As described earlier, when an ERT entry is invalidated, theassociated ERT_ID is invalidated, which also invalidates all associatedentries in the EAD table 300. Accordingly, an EAD hit implies an ERT hitbecause using the ERT_ID field 285, an ERT entry can be found for theload/store instruction. Accordingly, in case of the EAD hit, afteridentifying the corresponding EAD entry, the LSU reads out ERT_ID fromthe EAD entry and sends to SRQ, LMQ, and/or LRQF, as shown at 530. TheSRQ, LMQ, and/or LRQF use the EA from the EAD entry identified. In caseof store instructions, which use RA, the RA from the ERT entry is readout for L2 access, as shown at 540 and 545. Thus, because the RA is notused anywhere else but the store instructions, core implementing thetechnical solutions herein is called EA-only core.

Now consider the case where the instruction misses the EAD 290, that isthe EA of the instruction does not have a matching entry in the EADtable 300. The thread_id and EA are compared against each entry from theERT 255, as shown at 550. If an ERT hit occurs, that is an ERT entrymatches the parameters, the LSU reads out the RA (8:51) from the ERTentry, as shown at 555, and 530. For load requests the LSU sends the RAto the L2 cache for access, 530. For store instructions, the LSU storesthe RA in the SRQ and then sends the RA to the L2 cache when the storedrains to the L2 cache, as shown at 540-545.

If ERT miss occurs, the LSU initiates a reload for the ERT 255, as shownat 555 and 560. Further, ERT entry replacement is initiated. The ERTentry replacements are LRU based, and the LSU ensures to track synonymswithin OoO window during this process.

Thus, by implementing the above method for a load, if there is an EA-hitin the EA-based L1 directory, then no address translation is performed.This improves on the typical processor where, the L1 directory isRA-based, which in case of a miss at the L1 directory for a load, causesthe EA to be sent to an ERAT table for translation to get the RA that issent to the L2 directory and beyond.

Further, for stores, with the method described herein the LSU has to gothrough the ERT table to determine the RA that is then stored in theSRQR to drain down to the caches (L1, L2, memory) when the store getsdrained out of the SRQ. The SRQR holds all the RA for the stores. The RAis only stored for draining to the Nest (that is, L2, memory, and otherunits of the memory subsystem). The RA is not used for load-hit-store,store-hit-load or load-hit-load type of OoO execution hazard detectionas it is done in the typical solutions. The RA calculation for storeshappens before the store is completed, because after completion the LSUis unable to process any interrupt for the store (store can generate anaddress translation related interrupt, which is to be processed beforethe store is completed). Here the RA calculation is done when the storeis issued (from the SRQR), thus preventing the LSU from having toperform the address translation. Thus, stores gets issued and executedOoO, and then get completed in-order, and subsequently the stores getdrained from the SRQ in-order. Until a store is drained, no other threador core knows about the store (only the current thread knows). After thestore is drained from the SRQ, it is written into the L1 (if the linealready exists in L1) and L2 caches (if caching is enabled) and at thatpoint the store is known to all other threads and cores in the system100.

For instruction fetches that miss the EA-based L1 I-Cache, the EA istranslated to RA using the ERT 255 and the RA is sent to the Nest tofetch the I-Cache line. Here, the LHS (load-hit-store), SHL(store-hit-load) and LHL (load-hit-load) are all determined based on theEA and the ERT-Index stored with the directory entries in the EA-basedL1 cache (EAD 290). All entries in the EAD table 300 have theirtranslation valid in the ERT table 400, which can be used once LHS, SHL,and LHL are determined. If an ERT entry is invalidated, then thecorresponding L1 cache entries are invalidated.

The LRQF, which is the load-reorder-queue, ensures that all loadoperations are tracked from dispatch to complete. When a load isrejected (for cache miss, or translation miss, or previous instructionit depends on got rejected) the load is taken out of the issue queue andplaced in the LRQE for it to be re-issued from there.

FIG. 6 illustrates a flowchart for a method for reloading the ERTaccording to one or more embodiments of the present invention. An ERTreload causes an entry in the ERT to be created or updated in responseto and based on the ERT miss. The ERT receives the RA which is to beadded into the ERT 255 and compares the RA with each entry in the ERT0and ERT1, as shown at 605. If the RA does not exist in the ERT 255, andif a new entry can be created, the ERT 255 creates a new entry with anew ERT_ID to store the RA, as shown at 610 and 615. The new entry iscreated in either ERT0 or ERT1 based on the executing thread being thefirst thread or the second thread, respectively. In case the processoris operating in ST mode, the ERT0 is updated. If the ERT 255 does nothave an open slot for a new entry, an existing entry is replaced basedon least recently used, or other such techniques, as shown at 615.

If the existing entry(ies) in the ERT 255 is found with the same RA asthe received RA (reloading RA), the ERT 255 compares page size(0:1) ofthe existing entry(ies) with that for the received RA, as shown at 620.If the page size of the existing entries is smaller than that for thereloading RA, the existing entries for that RA are removed from the ERT255, and a new entry with a new ERT_ID is added for the RA with thelarger page size, as shown at 625. If the existing entry has a same orlarger page size, and if the implementation uses an SDT, an entry iscreated in the SDT for the reloading RA, as shown at 627. It should benoted that this operation may not be performed in case the LSU is usingan ERTE.

If the page size of the existing entries is the same size as thereloading RA, the ERT 255 checks if the existing entry is on the localERT for the executing thread, as shown at 630. A local ERT in this caserefers to the ERT being associated with the thread that is beingexecuted, for example ERT0 for first thread and ERT1 for a secondthread. If the RA hit is in the other ERT, that is the ERT that is notthe local ERT, the ERT 255 creates a new entry in the local ERT withERT_ID matching that in the non-local ERT, as shown at 632. For example,if the RA hit is in ERT1 for instruction executing by thread-0, an entryis created in the ERT0 with matching ERT_ID as the entry in the ERT1.

If the RA hit is on the local ERT instance, and if the EA also matches,because both EA and RA matche with an existing entry, but there was aERT miss for this thread prompting the ERT reload, the ERT deems thatthis indicates that the two threads are sharing the same EA-RA mapping(with the same page size). Accordingly, the tid_en(0) or tid_en(1) bitin the existing matching entry for the bit corresponding to the reloadthread is turned ON to indicate this case, as shown at 634.

If the RA hit is on local ERT instance, the EA does not match theexisting entry, and if the existing entry is for the same thread as thereloading RA, the ERT identifies the aliasing case where two differentEA maps to the same RA from the same thread, as shown at 636. If theprocessor is using an SDT-based implementation, a synonym entry isinstalled in the SDT that maps to the ERT ID, EA Offset(40:51) of theexisting matching entry. If the processor uses an ERTE-basedimplementation, the LSU rejects the instruction until it isnon-speculative, upon which it evicts the entry from the ERT and adds anentry in the ERTE.

If the RA hit is on the local ERT instance, the EA does not match theexisting entry, and if the existing entry is for a different thread, theERT identifies the aliasing case where two EAs map to same RA fromdifferent threads, as shown at 638. If the processor is using anSDT-based implementation, a synonym entry is installed in the SDT thatmaps to the ERT ID, EA Offset(40:51) of the existing matching entry. Ifthe processor uses an ERTE-based implementation, a new local ERT entryis added using a new ERT ID, with tid_en valid only for the thread thathad the ERT miss.

The above method facilitates that in the ERTE-based implementation, twothreads that have the same RA but different EA use two different ERTentries; and in SDT-based implementation, when two threads have the sameRA but different EA, one of the translation uses the ERT entry and theother will use the SDT entry. Thus, the ERT entries facilitate the casewhere the same EA, and same RA is used across different threads byhaving a tid_en field in the ERT entry. For example, Tid_en(0:1)={tid 0en, tid 1 en} on ERT0 instance; and Tid_en(0:1)={tid 1 en, tid 1 en} onERT1 instance. Further, the ERT entry facilitates the case where same EAcorrespond to different RAs across different threads by having multipleentries in the ERT0 and ERT1 with their respective thread identifiers.The ERT entries also support the case with different EAs correspondingto the same RA (either same or different thread cases). Two cases arenow described based on whether the implementation uses ERTE or SDT, bothare described further.

In case the LSU uses the implementation using SDT, the LSU installs anentry in the SDT (synonym detection table) instead of the ERT 255 when anew instruction is detected with different EA corresponding to the sameRA at ERT reload time. The SDT hits relaunch with the original (orearlier) ERT entry's EA. If new synonym page size is bigger than thepage size in the existing ERT entry with matching RA, then the existingERT entry is replaced by the new synonym (with the larger page size)instead of installing a synonym in the SDT. The old ERT entry iseventually reinstalled as synonym in the SDT.

Alternatively, in case the LSU uses the implementation using ERTE, ifthe instructions with different EAs corresponding to the same RA are fordifferent threads then the LSU install a new entry in the ERT table withthe appropriate Tid_en enabled. If the instructions are for the samethread, then the LSU rejects the load/store until it is non-speculative.After that point, the LSU removes the existing ERT entry and places itin the ERTE table, tagged with the ITAG of the youngest instructionin-flight from the thread. The LSU further installs the new EA-RA pairin the ERT table 400. This ensures that a situation when two differentEAs map to the same RA from the same thread does not occur.

Further, referring back to the ERT cases, consider the case where theLSU receives a snoop from another core from the processor 106. The snoopcan be from a different core in the system (snoop indiecates anothercore or thread, changed the data at the same real address). The LSU alsochecks stores from a thread within the core as a potential snoop to theother threads within the core. All snoops (from other cores) or stores(from other threads within the core) comes with a RA. In such cases, theLSU reverse translates the RA to determine corresponding EA, ERT_ID, andpage size based on the ERT 255. The LSU compares this information withthe ERT_ID, PS, EA(40:56) stored in each of the following structures todetect a snoop hit and take the appropriate action. For example, if asnoop hit is detected in LRQF Entry, the LSU indicates a potentialload-hit-load out of order hazard. If a snoop hit is detected in the EAD290, the LSU initiates an L1 invalidate, if the snoop is from adifferent core. If the store is from another thread for a shared line,then the line automatically gets the new store and is updated.

If the LSU uses an SDT (as opposed to an ERTE), and if a snoop hitexists in the LMQ, the LSU also updates the LMQ entry to not store in L1Dcache, the SRQ Entry is not used for snoops in SRQ, only used for LHSEA miss RA hit style checking, and a new SDT entry is created for thesnoop hit.

FIG. 7 illustrates a flowchart of an example method for hazard detectionof OoO execution of load and store instructions in processors withoutusing real addresses according to one or more embodiments of the presentinvention. The LSU has identified a load/store instruction referencingan EA (Load, store, LARX, DCB) to be issued, as shown at 605.Determining the instruction to be issued is based on the operationdescribed herein (FIG. 6). The instruction references the EA that isprovided by the computer program that is being executed by the system100, which has not yet been translated into the corresponding RA for theinstruction.

The LSU checks if there is an entry in the LRQF 218 with a matching pairof EA and ERT_ID as the instruction to be issued, as shown at 610. If amatching entry does not exist, there is no SHL/LHL hazard, because anolder load instruction referencing the same EA and ERT_ID pair does notexist in the LRQF 218. Therefore, the LSU proceeds to issue theinstruction, as shown at 630. For example, if the instruction is a loadinstruction, a corresponding entry is added into the LRQF 218.Alternatively, if the instruction is a store instruction, acorresponding entry is added into the SRQ 220.

If the LSU finds a matching entry in the LRQF 218, a flush message issent to the IFU 106 for the load/store instruction, as shown at 620. Theflush message includes an identifier for the instruction to be flushed,for example an itag of the instruction is included in the message. Theflush message causes the IFU 106 to fetch another instruction in OoOprocessing, where the instruction that is flushed is reloadedsubsequently in the future.

FIG. 8 illustrates a flowchart of an example method for hazard detectionof OoO execution of load and store instructions in processors withoutusing real addresses according to one or more embodiments of the presentinvention. The LSU has identified a load/store instruction referencingan EA (Load, store, LARX, DCB) to be issued, as shown at 705.Determining the instruction to be issued is based on the operationdescribed herein (FIG. 6). The instruction references the EA that isprovided by the computer program that is being executed by the system100, which has not yet been translated into the corresponding RA for theinstruction.

The LSU checks if there is an entry in the LRQF 218 with a matching pairof EA and ERT_ID as the instruction to be issued, as shown at 710. If amatching entry does not exist, there is no SHL/LHL hazard, because anolder load instruction referencing the same EA and ERT_ID pair does notexist in the LRQF 218. Therefore, the LSU proceeds to issue theinstruction, as shown at 730. For example, if the instruction is a loadinstruction, a corresponding entry is added into the LRQF 218.Alternatively, if the instruction is a store instruction, acorresponding entry is added into the SRQ 220.

If the LSU finds a matching entry in the LRQF 218, a flush message issent to the IFU 106 for the load instruction from the LRQF 218 and allsubsequent instruction that have been issued since that loadinstruction, as shown at 720. Alternatively, or in addition, if the LSUfinds a matching entry in the LRQF 218, a flush message is sent to theLRQF 218 for the load instruction from the LRQF 218 and all subsequentload instructions that have been issued since that load instruction.

The flush message includes an identifier for the load instruction fromwhere the IFU 106 is to be flushed. For example, an itag of the loadinstruction (from the LRQF entry) is included in the flush message. Incase the flush message is sent to the IFU 106, the flush message causesthe IFU 106 to fetch another instruction in OoO processing, where theinstruction that is flushed is reloaded subsequently in the future. Incase the flush message is sent to the LRQF 218, the flush message causesthe LRQF 218 to remove the corresponding entries from the LRQF 218, andinstead add the entries into an load relaunch queue (LRQE). After apredetermined amount of time, the entries from the LRQE are relaunched.

The technical solutions described herein thus facilitate using only theEA, providing technical advantages such that an ERAT (which wastypically used in processors) is not referred to in a load/store path,and further that detecting hazards, such as SHL/LHL, and suppressingDVAL in time do not cause timing problems. Specifically, in an SHL, ayounger load executes first and then a store of the same addressexecutes and queries the load reorder queue to trigger a flush of theyounger load. The technical solutions described herein address technicalchallenges using only the EA, for example that LHS, SHL, LHL detectioncan miss when two different EA map to the same RA. The technicalsolutions described herein address such technical problem by using theEA in conjunction with an ERT-index (ERT_ID) to make such detections bycomparing an issued load/store instruction with entries in an LRQF datastructure. The technical solutions provide various technical advantagesincluding reduction in chip area (by not storing RA), reduction in powerconsumption (by not translating EA-RA), and improvements in latency,among others.

Turning now to FIG. 9, a block diagram of a computer system 900 forimplementing some or all aspects of one or more embodiments of thepresent invention. The processing described herein may be implemented inhardware, software (e.g., firmware), or a combination thereof. In anexemplary embodiment, the methods described may be implemented, at leastin part, in hardware and may be part of the microprocessor of a specialor general-purpose computer system 900, such as a mobile device,personal computer, workstation, minicomputer, or mainframe computer.

In an exemplary embodiment, as shown in FIG. 9, the computer system 900includes a processor 905, memory 912 coupled to a memory controller 915,and one or more input devices 945 and/or output devices 947, such asperipherals, that are communicatively coupled via a local I/O controller935. These devices 947 and 945 may include, for example, a printer, ascanner, a microphone, and the like. A conventional keyboard 950 andmouse 955 may be coupled to the I/O controller 935. The I/O controller935 may be, for example, one or more buses or other wired or wirelessconnections, as are known in the art. The I/O controller 935 may haveadditional elements, which are omitted for simplicity, such ascontrollers, buffers (caches), drivers, repeaters, and receivers, toenable communications.

The I/O devices 947, 945 may further include devices that communicateboth inputs and outputs, for instance disk and tape storage, a networkinterface card (NIC) or modulator/demodulator (for accessing otherfiles, devices, systems, or a network), a radio frequency (RF) or othertransceiver, a telephonic interface, a bridge, a router, and the like.

The processor 905 is a hardware device for executing hardwareinstructions or software, particularly those stored in memory 912. Theprocessor 905 may be a custom made or commercially available processor,a central processing unit (CPU), an auxiliary processor among severalprocessors associated with the computer system 900, a semiconductorbased microprocessor (in the form of a microchip or chip set), amicroprocessor, or other device for executing instructions. Theprocessor 905 can include a cache such as, but not limited to, aninstruction cache to speed up executable instruction fetch, a data cacheto speed up data fetch and store, and a translation look-aside buffer(TLB) used to speed up virtual-to-physical address translation for bothexecutable instructions and data. The cache may be organized as ahierarchy of more cache levels (L1, L2, etc.).

The memory 912 may include one or combinations of volatile memoryelements (e.g., random access memory, RAM, such as DRAM, SRAM, SDRAM,etc.) and nonvolatile memory elements (e.g., ROM, erasable programmableread only memory (EPROM), electronically erasable programmable read onlymemory (EEPROM), programmable read only memory (PROM), tape, compactdisc read only memory (CD-ROM), disk, diskette, cartridge, cassette orthe like, etc.). Moreover, the memory 912 may incorporate electronic,magnetic, optical, or other types of storage media. Note that the memory912 may have a distributed architecture, where various components aresituated remote from one another but may be accessed by the processor905.

The instructions in memory 912 may include one or more separateprograms, each of which comprises an ordered listing of executableinstructions for implementing logical functions. In the example of FIG.9, the instructions in the memory 912 include a suitable operatingsystem (OS) 911. The operating system 911 essentially may control theexecution of other computer programs and provides scheduling,input-output control, file and data management, memory management, andcommunication control and related services.

Additional data, including, for example, instructions for the processor905 or other retrievable information, may be stored in storage 927,which may be a storage device such as a hard disk drive or solid statedrive. The stored instructions in memory 912 or in storage 927 mayinclude those enabling the processor 905 to execute one or more aspectsof the dispatch systems and methods of this disclosure.

The computer system 900 may further include a display controller 925coupled to a display 930. In an exemplary embodiment, the computersystem 900 may further include a network interface 960 for coupling to anetwork 965. The network 965 may be an IP-based network forcommunication between the computer system 900 and an external server,client and the like via a broadband connection. The network 965transmits and receives data between the computer system 900 and externalsystems. In an exemplary embodiment, the network 965 may be a managed IPnetwork administered by a service provider. The network 965 may beimplemented in a wireless fashion, e.g., using wireless protocols andtechnologies, such as WiFi, WiMax, etc. The network 965 may also be apacket-switched network such as a local area network, wide area network,metropolitan area network, the Internet, or other similar type ofnetwork environment. The network 965 may be a fixed wireless network, awireless local area network (LAN), a wireless wide area network (WAN) apersonal area network (PAN), a virtual private network (VPN), intranetor other suitable network system and may include equipment for receivingand transmitting signals.

Systems and methods for hazard detection of OoO execution of load andstore instructions in processors without using real addresses can beembodied, in whole or in part, in computer program products or incomputer systems 900, such as that illustrated in FIG.9.

Various embodiments of the invention are described herein with referenceto the related drawings. Alternative embodiments of the invention can bedevised without departing from the scope of this invention. Variousconnections and positional relationships (e.g., over, below, adjacent,etc.) are set forth between elements in the following description and inthe drawings. These connections and/or positional relationships, unlessspecified otherwise, can be direct or indirect, and the presentinvention is not intended to be limiting in this respect. Accordingly, acoupling of entities can refer to either a direct or an indirectcoupling, and a positional relationship between entities can be a director indirect positional relationship. Moreover, the various tasks andprocess steps described herein can be incorporated into a morecomprehensive procedure or process having additional steps orfunctionality not described in detail herein.

The following definitions and abbreviations are to be used for theinterpretation of the claims and the specification. As used herein, theterms “comprises,” “comprising,” “includes,” “including,” “has,”“having,” “contains” or “containing,” or any other variation thereof,are intended to cover a non-exclusive inclusion. For example, acomposition, a mixture, process, method, article, or apparatus thatcomprises a list of elements is not necessarily limited to only thoseelements but can include other elements not expressly listed or inherentto such composition, mixture, process, method, article, or apparatus.

Additionally, the term “exemplary” is used herein to mean “serving as anexample, instance or illustration.” Any embodiment or design describedherein as “exemplary” is not necessarily to be construed as preferred oradvantageous over other embodiments or designs. The terms “at least one”and “one or more” may be understood to include any integer numbergreater than or equal to one, i.e. one, two, three, four, etc. The terms“a plurality” may be understood to include any integer number greaterthan or equal to two, i.e. two, three, four, five, etc. The term“connection” may include both an indirect “connection” and a direct“connection.”

The terms “about,” “substantially,” “approximately,” and variationsthereof, are intended to include the degree of error associated withmeasurement of the particular quantity based upon the equipmentavailable at the time of filing the application. For example, “about”can include a range of ±8% or 5%, or 2% of a given value.

For the sake of brevity, conventional techniques related to making andusing aspects of the invention may or may not be described in detailherein. In particular, various aspects of computing systems and specificcomputer programs to implement the various technical features describedherein are well known. Accordingly, in the interest of brevity, manyconventional implementation details are only mentioned briefly herein orare omitted entirely without providing the well-known system and/orprocess details.

The present invention may be a system, a method, and/or a computerprogram product. The computer program product may include a computerreadable storage medium (or media) having computer readable programinstructions thereon for causing a processor to carry out aspects of thepresent invention.

The computer readable storage medium can be a tangible device that canretain and store instructions for use by an instruction executiondevice. The computer readable storage medium may be, for example, but isnot limited to, an electronic storage device, a magnetic storage device,an optical storage device, an electromagnetic storage device, asemiconductor storage device, or any suitable combination of theforegoing. A non-exhaustive list of more specific examples of thecomputer readable storage medium includes the following: a portablecomputer diskette, a hard disk, a random access memory (RAM), aread-only memory (ROM), an erasable programmable read-only memory (EPROMor Flash memory), a static random access memory (SRAM), a portablecompact disc read-only memory (CD-ROM), a digital versatile disk (DVD),a memory stick, a floppy disk, a mechanically encoded device such aspunch-cards or raised structures in a groove having instructionsrecorded thereon, and any suitable combination of the foregoing. Acomputer readable storage medium, as used herein, is not to be construedas being transitory signals per se, such as radio waves or other freelypropagating electromagnetic waves, electromagnetic waves propagatingthrough a waveguide or other transmission media (e.g., light pulsespassing through a fiber-optic cable), or electrical signals transmittedthrough a wire.

Computer readable program instructions described herein can bedownloaded to respective computing/processing devices from a computerreadable storage medium or to an external computer or external storagedevice via a network, for example, the Internet, a local area network, awide area network and/or a wireless network. The network may comprisecopper transmission cables, optical transmission fibers, wirelesstransmission, routers, firewalls, switches, gateway computers and/oredge servers. A network adapter card or network interface in eachcomputing/processing device receives computer readable programinstructions from the network and forwards the computer readable programinstructions for storage in a computer readable storage medium withinthe respective computing/processing device.

Computer readable program instructions for carrying out operations ofthe present invention may be assembler instructions,instruction-set-architecture (ISA) instructions, machine instructions,machine dependent instructions, microcode, firmware instructions,state-setting data, or either source code or object code written in anycombination of one or more programming languages, including an objectoriented programming language such as Java, Smalltalk, C++ or the like,and conventional procedural programming languages, such as the “C”programming language or similar programming languages. The computerreadable program instructions may execute entirely on the user'scomputer, partly on the user's computer, as a stand-alone softwarepackage, partly on the user's computer and partly on a remote computeror entirely on the remote computer or server. In the latter scenario,the remote computer may be connected to the user's computer through anytype of network, including a local area network (LAN) or a wide areanetwork (WAN), or the connection may be made to an external computer(for example, through the Internet using an Internet Service Provider).In some embodiments, electronic circuitry including, for example,programmable logic circuitry, field-programmable gate arrays (FPGA), orprogrammable logic arrays (PLA) may execute the computer readableprogram instructions by utilizing state information of the computerreadable program instructions to personalize the electronic circuitry,in order to perform aspects of the present invention.

Aspects of the present invention are described herein with reference toflowchart illustrations and/or block diagrams of methods, apparatus(systems), and computer program products according to embodiments of theinvention. It will be understood that each block of the flowchartillustrations and/or block diagrams, and combinations of blocks in theflowchart illustrations and/or block diagrams, can be implemented bycomputer readable program instructions.

These computer readable program instructions may be provided to aprocessor of a general purpose computer, special purpose computer, orother programmable data processing apparatus to produce a machine, suchthat the instructions, which execute via the processor of the computeror other programmable data processing apparatus, create means forimplementing the functions/acts specified in the flowchart and/or blockdiagram block or blocks. These computer readable program instructionsmay also be stored in a computer readable storage medium that can directa computer, a programmable data processing apparatus, and/or otherdevices to function in a particular manner, such that the computerreadable storage medium having instructions stored therein comprises anarticle of manufacture including instructions which implement aspects ofthe function/act specified in the flowchart and/or block diagram blockor blocks.

The computer readable program instructions may also be loaded onto acomputer, other programmable data processing apparatus, or other deviceto cause a series of operational steps to be performed on the computer,other programmable apparatus or other device to produce a computerimplemented process, such that the instructions which execute on thecomputer, other programmable apparatus, or other device implement thefunctions/acts specified in the flowchart and/or block diagram block orblocks.

The flowchart and block diagrams in the Figures illustrate thearchitecture, functionality, and operation of possible implementationsof systems, methods, and computer program products according to variousembodiments of the present invention. In this regard, each block in theflowchart or block diagrams may represent a module, segment, or portionof instructions, which comprises one or more executable instructions forimplementing the specified logical function(s). In some alternativeimplementations, the functions noted in the block may occur out of theorder noted in the figures. For example, two blocks shown in successionmay, in fact, be executed substantially concurrently, or the blocks maysometimes be executed in the reverse order, depending upon thefunctionality involved. It will also be noted that each block of theblock diagrams and/or flowchart illustration, and combinations of blocksin the block diagrams and/or flowchart illustration, can be implementedby special purpose hardware-based systems that perform the specifiedfunctions or acts or carry out combinations of special purpose hardwareand computer instructions.

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to best explain theprinciples of the embodiments, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdisclosed herein.

What is claimed is:
 1. A computer-implemented method for executing oneor more out-of-order instructions by a processing unit, the methodcomprising: detecting, by a load-store unit, a load-hit-load (LHL) in anout-of-order execution of instructions based only on effective addressesby: determining an effective address associated with a first loadinstruction; and determining a second load instruction entry using saideffective address being present in a load reorder queue; and indicatingthat a LHL has been detected based at least in part on determining thatthe second load instruction entry using said effective address ispresent in the load reorder queue; and in response to detecting the LHL,flushing, by the load-store unit, instructions starting from the secondload instruction corresponding to the second load instruction entry inthe load reorder queue, wherein the flushing comprises sending, to aninstruction fetch unit, a flush message that comprises an identifier ofthe second load instruction, wherein the flushing comprisesrelinquishing launch of the instructions fetched by the instructionfetch unit starting from the second load instruction.
 2. Thecomputer-implemented method of claim 1, further comprising: detecting,by the load-store unit, a store-hit-load (SHL) in the out-of-orderexecution of instructions based only on the effective addresses by:determining the effective address associated with a store instruction;and determining whether the second load instruction entry using saideffective address is present in the load reorder queue; and indicatingthat a SHL has been detected based at least in part on determining thatsecond load instruction entry using said effective address is present inthe load reorder queue; and in response to detecting the SHL, flushinginstructions starting from the second load instruction.
 3. Thecomputer-implemented method of claim 1, wherein determining whether thesecond load instruction entry using said effective address is present inthe load reorder queue further comprises: determining whether effectivereal table index for a first load instruction entry and the second loadinstruction match each other.
 4. The computer-implemented method ofclaim 1, wherein the load reorder queue comprises a number ofpartitions, one partition for each load instruction issued concurrentlyby the load-store unit.
 5. The computer-implemented method of claim 1,further comprising maintaining, by the load-store unit, a first loadinstruction entry for the first load instruction in the load reorderqueue until completion of the first load instruction.
 6. Thecomputer-implemented method of claim 1, wherein determining whether thesecond load instruction entry using said effective address is present inthe load reorder queue further comprises: determining whether effectivereal table index for the second load instruction entry and the storeinstruction match each other.
 7. A processing unit for executing one ormore instructions, the processing unit comprising: a load-store unit fortransferring data between memory and registers, the load-store unitconfigured to: detect a load-hit-load (LHL) in the out-of-orderexecution of instructions based only on effective addresses, whereindetecting the LHL comprises: determining an effective address associatedwith a first load instruction; determining whether a second loadinstruction entry using said effective address is present in the loadreorder queue; and indicating that a LHL has been detected based atleast in part on determining that second load instruction entry usingsaid effective address is present in the load reorder queue; and inresponse to detecting the LHL, flush instructions starting from thesecond load instruction, wherein the flushing comprises sending, to aninstruction fetch unit, a flush message that comprises an identifier ofthe second load instruction, wherein the flushing comprisesrelinquishing launch of the instructions fetched by the instructionfetch unit starting from the second load instruction.
 8. The processingunit of claim 7, wherein the load-store unit is further configured to:detect a store-hit-load (SHL) in an out-of-order execution ofinstructions based only on effective addresses by: determining aneffective address associated with a store instruction; determiningwhether a load instruction entry using said effective address is presentin a load reorder queue; and indicating that a SHL has been detectedbased at least in part on determining that load instruction entry usingsaid effective address is present in the load reorder queue; and inresponse to detecting the SHL, flush instructions starting from a loadinstruction corresponding to the load instruction entry.
 9. Theprocessing unit of claim 8, wherein the load-store unit is furtherconfigured to maintain the load instruction entry for the loadinstruction in the load reorder queue until completion of the loadinstruction.
 10. The processing unit of claim 8, wherein determiningwhether the load instruction entry using said effective address ispresent in the load reorder queue further comprises: determining whetheran effective real table index for the load instruction entry and thestore instruction match each other.
 11. The processing unit of claim 7,wherein determining whether the second load instruction entry using saideffective address is present in the load reorder queue furthercomprises: determining whether an effective real table index for thefirst load instruction entry and the second load instruction match eachother.
 12. The processing unit of claim 7, wherein the load reorderqueue comprises a number of partitions, one partition for each loadinstruction issued concurrently by the load-store unit.
 13. Theprocessing unit of claim 7, wherein the load-store unit is furtherconfigured to maintain a first load instruction entry for the first loadinstruction in the load reorder queue until completion of the first loadinstruction.
 14. The processing unit of claim 7, wherein determiningwhether the second load instruction entry using said effective addressis present in the load reorder queue further comprises: determiningwhether effective real table index for the second load instruction entryand the store instruction match each other.
 15. A computer programproduct comprising a computer readable storage medium having programinstructions embodied therewith, the program instructions executable bya processor to cause the processor to perform operations comprising:detecting, by a load-store unit, a load-hit-load (LHL) in anout-of-order execution of instructions based only on effective addressesby: determining an effective address associated with a first loadinstruction; and determining a second load instruction entry using saideffective address being present in a load reorder queue; and indicatingthat a LHL has been detected based at least in part on determining thatthe second load instruction entry using said effective address ispresent in the load reorder queue; and in response to detecting the LHL,flushing, by the load-store unit, instructions starting from the secondload instruction corresponding to the second load instruction entry inthe load reorder queue, wherein the flushing comprises sending, to aninstruction fetch unit, a flush message that comprises an identifier ofthe second load instruction, wherein the flushing comprisesrelinquishing launch of the instructions fetched by the instructionfetch unit starting from the second load instruction.
 16. The computerprogram product of claim 15, wherein checking if the second loadinstruction entry using said effective address is present in the loadreorder queue further comprises: checking if effective real table indexfor a first load instruction entry and the second load instruction matcheach other.
 17. The computer program product of claim 15, whereindetermining whether the second load instruction entry using saideffective address is present in the load reorder queue furthercomprises: determining whether effective real table index for the secondload instruction entry and the store instruction match each other.